Bicycle Mechanics - Fixing a loose handlebar stem???

I recently bought a 20" flatspin freestyle bike for my 6-year old. He loves it, but to me the handlebar stem seems to be a little loose. I have tightened the nut, but I am leery about stripping it. Someone mentioned putting a small piece of inner-tube, or black tape around the stem, then tighten.

Help please. I want to be certain the bike is as safe as it can be for him.

Thanks.

I know it's probably a stupid question, but I know little about bikes myself, and I just Googled the keywords: freestyle bike forums. I figured someone here could help me.

If you know an answer please help.

There are two connections: bar to stem, stem into top tube, from your post I presume stem into top
tube. However both connections should be tight enough that when loose you have to push and wiggle the parts to get them together. Neither connection should have any play before the bolts are tightened
up. If there is play, it should be a lot less than would allow tape or rubber to be used, neither is a good
idea, both are too squishy. If you need a shim, which as noted you should NOT, it would be best to buy a feeler gage set and use which ever blade fits into the gap. Dissassemble the set up and pull the loose
part out enough to check for play first and to look at the condition of the parts: occult cracks especially.
Cracked part would be a disaster. Bar and stem both should be examined. Steve

Thank you very much. Yes, I am speaking of the stem into top tube. What do you mean by a "shim"? And as noted I should NOT need one?

Sorry for my ignorance.

By shim he means using something to make a non-fitting piece fit. On my bike, the rear light connection to the seat post would be way to large, so I have a rubber shim in there, to make it a tighter fit. However for a stem you should NOT need a shim.

The Flatspin is an inexpensive Chinese bike. While a shim should not be necessary, the manufacturing tolerances may make one so. Stem shims are not necessarily dangerous - in fact some stems come with them to use 1 1/8"stems on 1" steerers. Rubber, tape, or anything compressible is a bad idea. The classic low budget shim is a strip of coke can. You can cut it to size with scissors. If you need more than 1-2 thicknesses to get it tight I would say the stem may be streched, cracked or otherwise damaged.

Thanks everyone. It is quite possible that the nut is simply not tightened enough. I was just cautious of overtightening, for fear that I could cause stripping. Should I crank it some more? I'm sure I could if necessary.